She went to the vet for a checkup/followup yesterday (December 18th) and the vet was very surprised at how well and how much she has recovered. She's definitely a strong little girl and it looks as if there won't be any permanent injury. She won't need any drains and they gave us a few more pain pills, but I don't think she will need many more. She is already chomping at the bit to get out and play, but we will be limiting her activity for another week.
I have been slowly integrating her back in to the pack, but there will never be a time when she is free-roaming with the three girls, only one at a time and only under direct supervision by me personally. In the plans are to create another dog yard that she can roam freely in without the three girls she has issues with.
I believe I have a good picture of what may have happened now. Zena and Velvet always play together, and sometimes it gets a bit rough. Velvet had a bruised and cracked rib where there were no bite marks, so I believe she was tackled on the hill and may have fallen on the rib and cracked it. Knowing she was hurt, she snapped at Zena to get her to stop playing, and that set off Moony and Ace. It is the best and most reasonable explanation, but a reminder that things happen that we can't always control and won't always be there for. The best we can do is minimize the risk.